Hi Jonny. you should never count the return pump as the turnover rate of the aquarium. i run around 500-1000 LPH through my tank from my sump.
it depends what you want to run off the pump and how good quality it is,
5000lph would do
-return to tank
-chiller
-skimmer or FBF
3000 LPH
is fine for just the return and chiller